In a mixing bowl, whisk together flour, cornstarch, baking soda, baking powder, and salt. In a separate bowl, beat unsalted butter, brown sugar, and granulated sugar until fluffy, approximately 3-4 minutes. Add peanut butter, vanilla extract, and egg, mixing until combined. Gradually incorporate dry ingredients, followed by quick oats, mini chocolate chips, and mini M&M's. Chill the cookie dough in the refrigerator for 40 minutes to firm it up.

Preheat your oven to 325°F (163°C). Grease a 9x9 inch baking pan and line it with parchment paper for easy removal. In a bowl, whisk together flour, cocoa powder, cornstarch, and salt, then set aside. In another bowl, beat together granulated sugar and eggs until frothy, about 2-3 minutes. Mix in melted butter, oil, and vanilla until fully combined. Fold in the dry mixture and semi-sweet chocolate chips until just blended.

Pour half of the brownie batter into the prepared pan, spreading it evenly. Bake in the preheated oven for 15 minutes, until the edges are set. While it bakes, flatten the chilled cookie dough into discs. Once the brownie layer is ready, carefully place the cookie dough discs on top, ensuring coverage. Pour the remaining brownie batter over the cookie dough, smoothing the top if necessary.

Return the pan to the oven and bake for an additional 20-30 minutes. You’ll know the Monster Cookie Fudgy Brownies are done when a toothpick inserted into the center comes out mostly clean, with a few moist crumbs attached. Avoid overbaking to maintain a fudgy texture; the edges should be firm but the center slightly soft.

Remove the brownies from the oven and allow them to cool completely at room temperature in the pan. This helps set the fudgy brownie texture and makes them easier to cut. Once cooled, lift the brownies out using the parchment paper and slice them into squares.